Within a wide field of activities, cranes and other hoists are being used to lift and move different work loads. The work loads are hooked on to a lifting hook or similar device. Via one or more wire ropes, the hook will then be connected to a winding drum, driven by an induction motor. Between the winding drum and the induction motor may be gear mechanisms in order to adapt suitable motor revolutions per minute to convenient lifting speed. The load will be lifted when the wire is wound on the winding drum and vice versa. The induction motor may advantageously be provided with slip rings connected to fixed or variable resistances. The purpose of this is to get a smooth starting of the lifting and to limit the starting currents of the motor. The hoist is dimensioned for the rated work load. In order not to exceed this load, the lifting should be switched off by to the hoist motor when the rated load is exceeded. Regulations about switches are available for example in statute-book AFS 1980:19 of the Swedish Board of Occupational Safety and Health. From .sctn. 13 is quoted: "A switch may be designed so as to time-delay or damp its switching-off function automatically when instantaneous excessive load occurs at the start. Normally, a time delay or damping must not last longer than 0.5 seconds."
This implies that the switch has to switch off the hoisting movement during a period when the system is still being exposed to acceleration forces, and before the start resistances have been uncoupled.
It is known in the art to estimate the work load at a speed of 0 or at a constant speed of movement upwards or downwards. For instance, this may be done by metering the tension in the wire ropes or the compressive forces of shafts and bearing supports.
It is also possible to estimate the work load weight by measuring the input power of the hoist motor, the electric current in the rotor winding, the current frequencies in the rotor winding, and so on. This is clear from the Russian patent publication No. 705270 and the Japanese patent publications No. 56-53420 and 56-58623. On the other hand, no simple method is known that is suitable continuously to estimate the weight of the load during starting up, during the acceleration and when the rotor resistances are switched on or off. No such method provides the hoist with an overload protection device that interrupts the hoisting at a very early stage, for example within half a second, so that no security risk will appear due to overload. The main difficulties arise on the one hand from the fact that energy is needed during acceleration for imparting kinetic energy to rotating parts and to the work load, the hook and other movable masses and on the other hand from the need of connecting and disconnecting various resistances in the rotor circuit.
Surprisingly, it has turned out that it is possible to estimate the weight of the load with great accuracy and reliability during the acceleration period at the start of a hoisting operation if the angular acceleration of the hoist motor rotor or some other rotary movement in the system is measured together with the input power of the motor and processed in a convenient computer unit.